The invention relates to antennas for radio frequency (RF) driven plasma ion sources.
An ion source is a plasma generator from which beams of ions can be extracted. Multi-cusp ion sources have an arrangement of magnets that form magnetic cusp fields to contain the plasma. Plasma can be generated in a multi-cusp ion source by DC discharge or RF induction discharge.
Unlike the filament DC discharge where eroded filament material can contaminate the chamber, RF discharges have a longer lifetime and cleaner operation. In an RF driven source, an induction coil or antenna placed into the ion source chamber is used for the discharge. However, there are problems with presently available RF antennas for plasma ion source applications.
U.S. Pat. No. 4,725,449 to Ellers et al. describes an RF antenna formed of a coil of copper tubing with a glass coating produced by applying a powdered glass frit, melting the frit to form a molten glass coating and cooling to solidify the glass to form a bonded glass coating on the tubing.
U.S. Pat. No. 5,434,353 to Kraus describes an RF antenna formed of a coiled quartz glass tube with a flexible wire inside the glass tube or with an electrically conducting layer precipitated on the inner wall of the tube.
U.S. Pat. No. 5,587,226 to Leung et al. describes an RF antenna formed of a tubular metal coil with a porcelain enamel coating bonded to the metal tubing.
The earliest RF antennas were made of bare conductors, but are subject to arcing and contamination. The bare antenna coils were then covered with sleeving material made of woven glass or quartz fibers or ceramic, but these were poor insulators. The glass or porcelain coated metal tubes were subject to differential thermal expansion between the coating and the conductor, which could lead to chipping and contamination. The glass tubes form good insulators for RF antennas, but in the design having a glass tube containing a wire or internal surface coating of a conductor, coolant flowing through the glass tube is subject to leakage upon breakage of the glass tube, thereby contaminating the entire apparatus in which the antenna is mounted with coolant.
Accordingly, it is an object of the invention to provide an improved RF antenna.
It is also an object of the invention to provide an RF antenna which has a glass tube insulator, but which is not subject to coolant leakage if the glass tube breaks.
It is another object of the invention to provide an RF antenna which has a metal tube coil disposed within a glass tube insulator.
It is a further object of the invention to provide a plasma ion source with an RF antenna which has a glass tube insulator, but which is not subject to coolant leakage if the glass tube breaks.
The invention is a radio frequency (RF) antenna for plasma ion sources. The RF antenna is formed of a metal tube disposed within a glass or quartz tube and formed into a coil. Thus the antenna is a metal tube coil inside a protective quartz tube. More generally the antenna is formed of a conducting tube, e.g. metal or alloy, within an insulating tube, e.g. glass or quartz.
The invention includes the method of making the RF antenna by first inserting the metal tube into the quartz tubing, and then heating the quartz tube with the inserted metal tube and bending the heated nested tubes into a coil. With this RF antenna design, coolant is circulated through the interior of the metal tubing so that breaks in the quartz tubing do not cause coolant loss.
The invention also includes a plasma ion source, typically a multi-cusp ion source, with an RF antenna formed of a metal tube coil inside a protective quartz tube.